In the Swedish patent SE 501 046 (application number 9300189-9), and the corresponding U.S. Pat. No. 5,621,621, the function of a self-oscillating converter according to prior art is described. The principal function for the converter is that the transformer T1 determine the self-oscillating frequency by entering a state of saturation every half-period. By providing a magnetic flow from the core T3, T1 will saturate faster which cause the converter frequency to increase. This cause, in turn, that the rectified outgoing voltage from the converter will decrease since the alternating voltage over C4, which constitutes a series resonance circuit together with the inductance L1, decreases at the same degree at the frequency increase. An advantage with this type of converter is that at high frequencies, more than 150 kHz, there will not be any fault with the driving to the transistors Tr1 and TR2, since the transformer T1, which delivers the drive pulses, sense the current through the transistors. The current through TR1 and TR2 transforms in T1 to a drive current for the transistors. The converter according to the Swedish patent SE 501 046 is well liked for its reliability in operation, simple construction and to safely operate at high frequencies.
In some cases it may however be troublesome that the frequency is dependent on when T1 saturates. This is true mainly when you want to be able to set the frequency very accurately. The properties of T1 such as saturation point is determined by sintering of its core and it is easy to understand that it becomes difficult to keep tight tolerances by such a rough mechanical process.